Metal-air batteries belong to batteries comprising an air electrode and use a metal negative electrode, based for example on zinc, iron or lithium, coupled to the air electrode. During discharge, the following reactions take place:M→Mn++n.e− negative electrode (metal electrode)O2+2.H2O+4.e−→4.OH− positive electrode (air electrode).
Thus, oxygen is reduced at the air electrode and the metal of the negative electrode is oxidized. Usually, an aqueous alkaline electrolyte is used.
These metal-air batteries have several applications, for example zinc-air batteries are marketed for use in auditory prostheses.
Numerous studies have been carried out over several decades for the development and optimization of air electrodes making it possible to produce electrochemical generators of the metal-air type, known for their high energy densities and capable of reaching several hundred Wh/kg.
Air electrodes are also used in alkaline fuel cells.
An air electrode makes it possible to use atmospheric air, which is available in unlimited quantity anywhere and at any time, as oxidizing agent for the electrochemical reaction.
An air electrode is a porous solid structure in contact with the liquid electrolyte, which is generally an alkaline solution. The interface between the air electrode and the liquid electrolyte is a so-called “triple contact” interface, in which the active solid material of the electrode, the gaseous oxidizing agent (air) and the liquid electrolyte are present simultaneously.
A description of the different types of air electrodes for zinc-air batteries is disclosed for example in the bibliographical article by V. Neburchilov et al., entitled “A review on air cathodes for zinc-air fuel cells”: Journal of Power Sources, 195 (2010), pages 1271 to 1291.
When a metal-air battery is to be electrically charged, the direction of the current is reversed and the following reactions take place:Mn++n.e−→M negative electrode (metal electrode)4.OH−→O2+2.H2O+4.e− positive electrode (air electrode)
Thus, oxygen is produced at the positive electrode and the metal is re-deposited by reduction on the negative electrode.
Although these batteries operate without a major problem in the discharge phase, they are not stable in the charging phase; the weak point of the metal-air battery during the charging phase is the air electrode, which is not designed to be used in the reverse direction (i.e. under oxidation).
In fact, the air electrode has a porous structure and operates in the form of a volumetric electrode in which the electrochemical reaction takes place within the volume of the electrode, at the interface between a gas (oxygen from the air), a liquid (the electrolyte) and a solid (the active material of the electrode and optionally a catalyst): this is the triple contact. This porous structure is important because it offers a necessary large reaction surface area, and therefore a high current density, as the density of the gaseous oxygen is low with respect to a liquid. For example, the molar density of the oxygen in air is equal to about 0.03 mol/L while water has a density of 55 mol/L.
Thus, generally, an air electrode is manufactured from carbon granules with a high surface area, such as Vulcan® XC72 marketed by Cabot. The surface area of the carbon can also be further increased by reaction with a gas, such as CO2, before its incorporation into the air electrode. The carbon granules are then agglomerated in order to form the air electrode, using a hydrophobic fluorinated polymer such as a fluorinated ethylene propylene copolymer (FEP) marketed by Dupont. Document WO 2000/036677 describes such an electrode for a metal-air battery.
This large reaction surface area is not necessary for the reverse oxidation reaction at the positive electrode during the charging phase, since the concentration of active material is much higher. On the contrary, the porous structure of the air electrode has the drawback of being fragile: it was found by the inventors that the porous structure of the air electrode was mechanically destroyed by the release of gaseous oxygen when it was used for oxidation of the liquid electrolyte to oxygen. In fact, the hydraulic pressure generated within the air electrode by the production of gaseous oxygen is sufficient to cause breaking of the bonds between the carbon granules constituting the air electrode.
The inventors also noted that the catalyst, which is added to the air electrode in order to improve the energy yield of the oxygen reduction reaction such as manganese or cobalt oxide, is not stable at the potential necessary for the oxygen reduction. Furthermore, corrosion takes place by oxidation of the carbon in the presence of oxygen and is accelerated at high potentials.
In order to overcome this, some authors use a more resistant oxygen reduction catalyst coupled with an oxygen release catalyst in the bifunctional electrodes composed of two electrically-coupled layers (see for example patent U.S. Pat. No. 5,306,579). Unfortunately, these bifunctional electrodes have a short lifetime and a limited number of cycles because the structure of these electrodes does not withstand the release of gas produced over long periods of time and because the catalyst is not stable and the carbon corrodes at the potentials applied during charging.
These degradations of the air electrode during the charging phase significantly reduce its lifetime and are one of the main reasons that prevent the commercial development of electrically rechargeable metal-air storage cells.
As a result, the lifetime of the air electrode is shorter than that of the metal electrode for batteries/cells used alternately in discharge and charge mode. Now, it would be a waste to have to discard the battery/cell when the metal electrode is still usable.
Generally, the problem associated with the release of gas during charging at the air electrode is found for any battery comprising an air electrode.